1. Field of Invention
The present invention relates generally to a structural framing system for double hull vessels and, more particularly, to a modular, unidirectional, longitudinally framed structural system for double hulled vessels having combinations of widely spaced transverse bulkheads, longitudinal bulkheads, and/or intermediate transverse web frames arranged to obtain a desired arrangement of cargo tanks or holds.
2. Brief Description of Related Art
Conventional framing systems for single hull bulk carrier vessels consist of a complex grillage of longitudinal framing members (typically spaced at 2 to 4 foot intervals) and closely spaced transverse framing members (typically spaced at 8 to 18 foot intervals depending on the length of the vessel). The complex grillage provides all the necessary structural support to withstand external sea pressures and internal liquid cargo pressures. Longitudinal and transverse bulkheads provide cargo and watertight subdivisions but are not required for structural support. Therefore, these bulkheads may be advantageously located to provide desired size and layout of cargo compartments. Conventionally framed double hull vessels include similar grillage between inner and outer hull shells. The principal disadvantage of conventional grillage framing systems is the complexity and high construction cost that result from the large number of individual structural members and the large extent of fit up and welding required.
Unidirectional framing systems for double hulled vessels, also known as "Advanced Double Hull" (ADH) vessels, were developed to simplify construction and increase producibility by reducing the number of structural members. Unidirectional framing systems consist of inner and outer hulls having only longitudinal support members (e.g., stringers and girders) therebetween that extend between transverse bulkheads, that is, they include no transverse support members (e.g., web frames) between transverse bulkheads. Advantages of double hulled, unidirectional framing systems include simplification of structure, improved resistance to collision and grounding, greater resistance to fatigue and failure, and reduced construction time and cost. However, the longitudinal cells formed by the inner hull, outer hull, and longitudinal support members require transverse support to withstand the applied external sea loads and internal cargo loads. Accordingly, known double hull vessels employ transverse bulkheads to provide the necessary transverse support.
In conventional grillage framed vessels, the pressure loads from both the sea and cargo act on the hull shell plating and are transmitted partly to the transverse frames and partly to the longitudinal stiffeners. These members then transmit the loads to the transverse and longitudinal bulkheads. In a unidirectional double hull configuration, the lateral loads on the shell plating are transmitted via bending to the longitudinal support members and then to the transverse bulkheads through shear. However, as distance between transverse bulkheads increases (i.e., as length of cargo compartments increases) the bending moments also increase. Under primary loading (i.e., hull girder bending), the plate stiffener collapse behavior of conventional grillage framing is replaced by the cellular column behavior of unidirectional double hull framing systems. There is little experience with accounting for such different structural behavior in present design methods. To ensure adequate structural strength, the thickness of hull, bulkhead, and/or longitudinal support member plating may be increased. However, this drastically increases the weight of the vessel. In practice, plating thickness of more than about 1.5 inches is impractical for surface vessels due to the inherent weight penalty.
As previously stated, double hull vessels employ transverse bulkheads to provide necessary transverse support. The high pressure loads usually experienced by large bulk carriers, e.g., very large crude carriers (VLCC's), limit the spacing of transverse bulkheads required to support the longitudinal cells. For unidirectional framed double hull vessels, the maximum distance between transverse bulkheads based on strength requirements, and thus the corresponding maximum cargo tank length, is approximately 50 feet. Consequently, an ADH-VLCC would have many more tanks in the longitudinal direction than would be found on conventional grillage framed vessels. Decreasing the size and increasing the number of tanks, when strength considerations require a large number of transverse bulkheads, results in operational difficulties that make ADH-VLCC's potentially unattractive to tanker operators. Consequently, a principal disadvantage of unidirectional framing systems is the structurally based limitation in length between supporting transverse bulkheads that, in turn, limits maximum cargo compartment length.
Therefore, with respect to large double hull vessel design, there is a present need of a means or method for providing transverse support while reducing the number of longitudinal tanks and increasing the lengths of longitudinal tanks without detrimentally affecting the structural strength, weight or cost of the vessel.